2004-2012, the results are heading in the same sharply downward direction for both "stock photography" and "stock photo". Interest in stock seems to have steeply declined since the high point of the debt fuelled 2000s.

Interest in "microstock" seems to have peaked 2009-2010 if these charts mean anything. Without any evidence I would argue that this is a much more nebulous, much less significant Google search term (since I suspect but have no evidence that it is one which people looking to buy images use much less).

I have no idea whether any of this is significant or means anything

Perhaps more definitively, check out the trend graph for Istockphoto;
... then compare and contrast it with the graph for Shutterstock;

Interesting. But why would you say that is more definitive ? Definitive in what sense or context ?

IS is typically and commonly known both as Istock and Istockphoto. I think that you need to combine the results for these two.

here are the Google trend comparisons for Getty + Istock + Istockphoto + shutterstock.

You can see that Getty + Istock + Istockphoto combined continue to be ahead of Shutterstock on the trends graph. The Shutterstock upward graph coincides with the period of the IPO and their heavy investment in advertising and other promotions.

The thing that this seems to show most of all is that the total interest in these companies is declining. Just as interest in "stock photography" is declining. If Google Trends means anything. Shutterstock does not appear to be significantly capturing the interest which Getty Images has let go.

Interesting. But why would you say that is more definitive ? Definitive in what sense or context ?

I mean 'definitive' as in 'a more closely defined question' leading, hopefully, to more accurate and useful results.

If you check out the Google trend on either 'stock images' or 'stock photos' you get similar graphs showing a peak in 2005 and gradually declining ever since. What does this mean? Well, as far as I understand it, it is just showing how many folk are searching on an extremely general search term __ folk who apparently know little about the industry or who the main agencies are. I would expect people in that situation to do their initial research and then choose a shop or two from which to buy their stuff.

When those folk next need more stuff, assuming that they'd had a favourable experience, I'd expect them to return to the same shop/s either via a bookmark or by a search for the specific shop. I wouldn't expect them to use a general search term again. The 'decline' you are noting in your OP may be simply indicating that those who require stock images are becoming more knowledgeable about the industry __ not that they have less interest in it.

Note that SS have been growing steadily from the start. That suggests to me that they have been particularly successful in satisfying their customers and ensuring that they return. Unlike Istock.

Could it be that with the introduction of Google Images, those in need of images don't need to search for stock anymore? Just a quick perusal of Google Images, grab what you need and Voila! Project complete. 

There is no doubt in my mind that the entire stock industry is on a down or rather after have been on a micro hype, its just settling down to shall we say normal.
The only area where I see an increase both in sales and price, is in the RM and thats probably down to old buyers, creative buyers, type ad-agencies etc who pretty much always buy RM. This is me, cant vouch for anybody else.